End ring and rotor bar for line start permanent magnet motor

ABSTRACT

A rotor for an LSIPM comprises a plurality of permanent magnets defining a number of poles (“P”) of the LSIPM, and a plurality of rotor bars spaced about the rotor defining a rotor bar area (“BA”). The rotor bars are formed of a conductive material having an associated conductivity (“σRB”). End members are disposed on axial opposite ends of the rotor core. The end members are in electrical contact with the rotor bars. The end members are formed from a material having an associated conductivity (“σEM”). Each end ring member has a minimum geometric cross sectional area (“ERA”) and outer diameter that generally corresponds to the rotor core outer diameter. The ERA is greater than 0.5 times the rotor bar area per the number of poles (BA/P) times a ratio of the rotor bar material conductivity to the end member material conductivity (σRB/σEM).

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of copending U.S. patentapplication Ser. No. 14/258,281, filed Apr. 22, 2014, which applicationis incorporated herein by reference.

BACKGROUND

Synchronous motors, including line start, interior permanent magnet(LSIPM) motors, are typically very efficient. A LSIPM motor will producetorque to accelerate from zero speed when started across the line, andthen operate as a synchronous motor with no rotor cage losses once fullyup to synchronous speed. However, synchronous motors have limitedcapability to pull into synchronism loads that have a high torque orhigh inertia. For certain applications, it is necessary for a LSIPM todemonstrate satisfactory starting performance in addition to thesteady-state performance. For a LSIPM motor, this includes more thanjust meeting rated starting current and starting torque during theasynchronous period of acceleration as would be the case for aninduction motor. The LSIPM motor must also be able to pull a load intosynchronism and achieve normal steady state operation. Both load torqueand load inertia are considerations whether a specific LSIPM motor willbe able to successfully start and synchronize a load. Accordingly, thebenefits in efficiency gains and energy savings ordinarily associatedwith synchronous motors are not typically achieved in applicationshaving loads with high inertia and/or high torque characteristics. Inthe past, an inverter has been used with synchronous motors in suchapplications to power the motor during starting. However, an inverteradds substantial costs and degrades system efficiency.

To achieve the steady state benefits of efficiency provided bysynchronous motors, and reduce limitations during start-up, rotor endrings and rotor bars may be designed to improve the ability of a motorto synchronize loads with higher torque and/or inertia requirementscompared with similar motors having conventional end ring and rotor bardesigns. The rotor end rings and rotor bars may be configured to reducefull load asynchronous slip by decreasing rotor resistance duringstart-up. While a decrease in rotor resistance may theoretically beachieved using induction motor principles (i.e., by increasing the totalcross sectional area of the rotor bars forming the starting cage),increasing the area of the rotor bars has a negative impact on thestarting and full load operating performance of the motor. For instance,increased rotor bar area results in increased flux density in the rotorand lower power factor, higher current, and more losses at full load,and higher locked rotor current at starting.

SUMMARY

This disclosure is directed to employing an end ring which has a largercross sectional area than would typically be used for the given bar areain order to reduce the asynchronous slip and improve loadsynchronization capability while not impacting the starting current orfull load performance of the machine running as a synchronous motor.This disclosure is also directed to employing a rotor cage formed frommaterials with favorable conductive properties in order to reduce theasynchronous slip and improve load synchronization capability while notimpacting the starting current or full load performance of the machinerunning as a synchronous motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is perspective view of an LSIPM.

FIG. 2 is a partial cross-section view of an electric motor taken fromlines 2-2 of FIG. 1.

FIGS. 3-6 show illustrative embodiments of laminations used in a rotorof the motor of FIG. 1.

FIG. 7 is an enlarged view of an end ring.

FIG. 8 is a cross-sectional view of the end ring taken from lines 8-8 ofFIG. 7.

FIG. 9 is a chart showing the slip and load inertia for a given end ringwidth for a LSIPM.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary LSIPM 10. The exemplary motor 10comprises a frame 12 capped at each end by drive and opposite drive endcaps 14,16, respectively. The frame 12 and the drive and opposite driveend caps 14,16 cooperate to form the enclosure or motor housing for themotor 10. The frame 12 and the drive and opposite drive end caps 14,16may be formed of any number of materials, such as steel, aluminum, orany other suitable structural material. The drive and opposite drive endcaps 14,16 may include mounting and transportation features, such as theillustrated mounting feet 18 and eyehooks 20.

To induce rotation of the rotor, current is routed through statorwindings disposed in the stator. (See FIG. 2). Stator windings areelectrically interconnected to form groups. The arrangement of thewindings in the stator core defines the phases associated with theLSIPM. The stator windings are further coupled to terminal leads (notshown), which electronically connect the stator windings to an externalpower source (not shown), such as 480 VAC three-phrase power or 110 VACsingle-phase power. A conduit box 24 houses the electrical connectionbetween the terminal leads and the external power source. The conduitbox 24 comprises a metal or plastic material, and advantageously,provides access to certain electrical components of the motor 10.Routing electrical current from its external power source through thestator windings produces a magnetic field that induces rotation of therotor. A rotor shaft 26 coupled to the rotor rotates in conjunction withthe rotor. That is, rotation of the rotor translates into acorresponding rotation of the rotor shaft 26. The rotor shaft may becoupled to any number of drive machine elements, thereby transmittingtorque to the given drive machine element. By way of example, machinessuch as pumps, compressors, fans, conveyors, and so forth, may harnessthe rotational motion of the rotor shaft 26 for operation.

FIG. 2 is a partial cross-section view of the motor 10 of FIG. 1 alongplane 2-2. To simplify the discussion, only the top portion of the motor10 is shown, as the structure of the motor 10 is essentially mirroredalong its centerline. As discussed above, the frame 12 and the drive andopposite drive end caps 14,16 cooperate to form an enclosure or motorhousing for the motor 10. Within the enclosure or motor housing residesa plurality of stator laminations 30 juxtaposed and aligned with respectto one another to form a lamination stack, such as a contiguous statorcore 32. In the exemplary motor 10, the stator laminations 30 aresubstantially identical to one another, and each stator lamination 30includes features that cooperate with adjacent laminations to formcumulative features for the contiguous stator core 32. For example, eachstator lamination 30 includes a central aperture that cooperates withthe central aperture of adjacent stator laminations to form a rotorchamber 34 that extends the length of the stator core 32 and that issized to receive a rotor. Additionally, each stator lamination 30includes a plurality of stator slots disposed circumferentially aboutthe central aperture. The stator slots cooperate to receive one or morestator windings 36, which are illustrated as coil ends in FIG. 2, thatextend the length of the stator core 32. As described in more detailbelow, upon start-up, the stator winding is energizable with analternating voltage to establish a rotating primary field that co-actswith the rotor bars of the rotor to start the rotor under inductionmotor principles.

In the exemplary motor 10, a rotor assembly 40 resides within the rotorchamber 34. Similar to the stator core 32, the rotor assembly 40comprises a plurality of rotor laminations 42 aligned and adjacentlyplaced with respect to one another. Thus, the rotor laminations 42cooperate to form a contiguous rotor core 44. When assembled, the rotorlaminations 42 cooperate to form a shaft chamber that extends throughthe center of the rotor core 44 and that is configured to receive therotor shaft 26 therethrough. The rotor shaft 26 is secured with respectto the rotor core 44 such that the rotor core 44 and the rotor shaft 26rotate as a single entity about a rotor center axis 45.

The exemplary rotor assembly 40 also includes electrically conductivemembers, such as rotor bars 48, disposed in the rotor core 44electrically connected to rotor end rings or end members 46 to form thestarting cage. The end rings or end members 46, which are disposed onopposite ends of the rotor core 44 are generally circular incross-section and have an outer diameter that generally approximates thediameter of the rotor laminations 42. The rotor bars 48 in cooperationwith the end rings 46 form at least one closed electrical pathway forinduced current within the rotor 40. Accordingly, the rotor bars 48 andthe end rings 46 comprise materials having good electrical conductivity,such as copper alloys as described below. Additional detail of the rotorbars and the end rings will be described in greater detail below.

To support the rotor assembly 40, the exemplary motor 10 includes driveand opposite drive bearing sets 50,52, respectively, that are secured tothe rotor shaft 26 and that facilitate rotation of the rotor assembly 40within the stationary stator core 32. During operation of the motor 10,the bearing sets 50,52 transfer the radial and thrust loads produced bythe rotor assembly 40 to the motor housing. Each bearing set 50,52includes an inner race 54 disposed circumferentially about the rotorshaft 26. The tight fit between the inner race 54 and the rotor shaft 26causes the inner race 54 to rotate in conjunction with the rotor shaft26. Each bearing set 50,52 also includes an outer race 56 and rotationalelements 58, which are disposed between the inner and outer races 54,56.The rotational elements 58 facilitate rotation of the inner races 54while the outer races 56 remain stationary and mounted with respect tothe drive and opposite drive end caps 14,16. Thus, the bearing sets50,52 facilitate rotation of the rotor assembly 40 while supporting therotor assembly 40 within the motor housing, i.e., the frame 12 and thedrive and opposite drive end caps 14,16. To reduce the coefficient offriction between the races 54,56 and the rotational elements 58, thebearing sets 50,52 are coated with a lubricant. Although the drawingsshow the bearing sets 50,52 with balls as rotational elements, thebearing sets may be other constructions, such as sleeve bearings, pinsbearings, roller bearings, etc.

FIGS. 3-6 provide further detail of illustrative embodiments of therotor laminations 42. Each rotor lamination 42 has a generally circularcross-section and is formed of a magnetic material, such as electricalsteel. Extending from end-to-end, i.e., transverse to the cross-section,each lamination 42 includes features that, when aligned with adjacentlaminations 42, form cumulative features that extend axially through therotor core 44. For example, each exemplary rotor lamination 42 has acircular shaft aperture 62 located in the center of the lamination 42.The shaft apertures 62 of adjacent laminations 42 cooperate to form ashaft chamber configured to receive the rotor shaft 26 (see FIG. 2)therethrough. The rotor core has an outer diameter (“D_(r)”).

Additionally, each lamination 42 includes a series of rotor bar slots 64that are arranged at positions about the lamination such that whenassembled, the rotor bar slots cooperate to form channels for the rotorbars that extend through the rotor core 44. The rotor bar slots arespaced radially inward from the rotor outer diameter (D_(r)). As shownin the drawings, each of the rotor bar slots may extend radially outwardto generally the same radial position relative to the rotor outerdiameter (D_(r)), or one or more rotor bar slots may extend radiallyoutward and terminate at different radial distances relative to theouter diameter (D_(r)), depending upon the application. The rotor bars48 may present the same shape as the rotor bar slots 64 to provide atight fit for the rotor bars 48 within the rotor channels. The rotorbars may be manufactured with tight tolerances between the rotor bars 48and the rotor bar slots, for instance, for a fabricated/swaged rotor bardesign.

Additionally, the rotor laminations 42 include magnet slots 70. Magnets72 may be disposed in the magnet slots in various ways to form poles forthe rotor. The magnet slots may be arranged so the magnets are in asingle layer or multi-layers. The magnet slots may also be arranged sothe magnets form a conventional “v”- or “u”-shape, or an inverted “v”-or “u”-shape. There may be only one magnet per slot or multiple magnetsper slot. The magnets may be magnetized in a generally radial directionto establish alternately inwardly and outwardly disposed north and southpoles on adjacent magnets. This means that adjacent magnets cooperate toestablish alternate north and south poles on the periphery of the rotor.The rotor may be constructed with any even number of poles. An exemplarylamination for a two pole motor is shown in FIG. 3, and exemplarylaminations for a four pole motor are shown in FIGS. 4-6. As shown inthe drawings by example and not in any limiting sense, the magnets mayestablish a direct axis as indicated by reference character 80 and aquadrature axis as indicated by reference character 82. The magnetsdefine a general axis of magnetization (north or south pole) on theperiphery of the rotor. The edges of the magnet slots facing the generalaxis of magnetization, which are radially outward from the magnets,establish a generally arcuate saturation boundary area as indicated byreference characters 84 a, 84 b. In cases, where a magnet is disposed inthe magnet slot, the edges of the magnet slots facing the general axisof magnetization and the edges of the magnets will be the same. FIGS. 3and 6 show embodiments where there is a gap 85 between the permanentmagnets in the magnet slots. In a multi-layer arrangement such as shownin FIG. 6, the saturation boundary area is defined by the magnet slotsthat are nested radially outward the farthest.

In each of the designs of the laminations shown in FIGS. 3-6, the magnetslots 70 extend to the peripheral edge of the rotor such that an end ofthe magnet slot is adjacent the peripheral edge. One or more of themagnet slots may have its radially outward end at generally the sameradial position relative to the rotor outer diameter (D_(r)) and therotor bar slots as shown in the drawings, or one or more magnet slotsmay extend radially outward and terminate at different distancesrelative to each other and/or the rotor bar slots, depending upon theapplication. The magnets 72 disposed in the magnet slots have a smallerlongitudinal length in the direction of the magnet slots than the magnetslots such that the magnet when installed in the magnet slot forms amagnet slot aperture 86 between the end of the permanent magnet and themagnet slot. The magnet slot aperture may be filled with conductivematerial to form additional rotor bars that are also connected to theend members 46.

The rotor bars 48 forming the starting cage may have a different size,shape, and spacing from rotor bars found in a machine having a uniformcage. Additionally, the rotor bar slots 64 may be distributed about therotor in a manner that is asymmetric rather than evenly distributed,i.e., asymmetric rather than equiangularly spaced, around the outer edgeof the lamination surface. Additionally, the rotor bar slots may have anarbitrary shape. The laminations may be stacked off-set to one anothersuch that the rotor bar in the slot has a helix relative to the rotoraxis of rotation. Additionally, a rotor bar slot 90 may be provided toalign with the quadrature axis 82. The rotor bar slot 90 of thequadrature axis may have a geometry which matches at least one of therotor bar slots aligned with the direct axis 80. Although some of thedrawings show a plurality of rotor bar slots in the direct axis and onerotor bar slot in the quadrature axis, other variations may be used. Therotor bar area (“BA”) is the cumulative area of all of the rotor barslots in a lamination that are intended to be filled with conductivematerial, including areas between magnets in the magnet slots, andincluding rotor bar slots provided in the quadrature axis and outside ofthe saturation boundary area.

The laminations shown in FIGS. 3-6 are configured to optimize paths forflux over a range of conditions including at rated load. In each of thelaminations shown in FIGS. 3-6, the arrangement of the starting cage ofthe rotor bars and the magnets allows for passage of rotor flux under awide range of loads and operating conditions. With each of the exemplarembodiments of FIGS. 3-6, the distance between the rotor bar slotsdisposed in the saturation boundary area 84 a,84 b and the magnet slotsis selected so that each rotor bar slot in the saturation boundary areamay be positioned away from an adjacent magnet slot by a distance thatequals or exceeds about four percent (4%) of the pole pitch. Accordingto another aspect of the present disclosure, the closest approachdistance of any one of the rotor bar slots in the saturation boundaryarea to an adjacent magnet slot may be about equal or exceed fourpercent of the pole pitch. The closest approach distance is referred tohereinafter as (“D_(rb-m)”) and is defined by the equation(“D_(rb-m)”)≥0.04×(“pp”). The pole pitch for the machine (“pp”) may bedefined by the equation (“pp”)={(“D_(R)”)×(π)}/(“P”), where “D_(R)” isthe diameter of the rotor and (“P”) is the number of poles for themachine as defined by the number of groups of permanent magnets. One ormore of the rotor bar slots in the saturation boundary area may bearranged to maintain this parameter relative to an adjacent magnet slot.Rotor bar slots outside of the saturation boundary area, for instance,rotor bar slots 90 generally aligned with the quadrature axis 82, mayalso be positioned to maintain this parameter relative to an adjacentmagnet slot.

In the rotor designs shown in FIGS. 3-6, at least one of the rotor barslots 64 in the saturation boundary area has a radial interior edge 92which conforms generally to a side of the magnet 72 in the adjacentmagnet slot 70. FIGS. 3-6 show the magnet arranged in the magnet slot invarious configurations. In each example, the interior radial edge of oneor more of the rotor bar slots 64 in the saturation boundary area has ageometry which generally matches the geometry of the magnet adjacent tothe rotor bar slot. One or more of the rotor bar slots in the saturationboundary area may be formed to have a radial inward edge which defines areference plane generally parallel to the adjacent magnet. In this way,one or more of the rotor bar slots may have a distance to the adjacentmagnet slot that meets or exceeds the four percent (4%) of the polepitch (“pp”). Rotor bar slots outside of the saturation boundary area,for instance, rotor bar slots 90 generally aligned with the quadratureaxis 82, may also be shaped in a similar manner to maintain thisparameter.

Referring to FIGS. 7 and 8, each of the end rings 46 comprises anannular disk with generally flat faces 100,102 and an outer diametersurface 104 and an inner diameter surface 106. The end ring outerdiameter surface 104 has an outer diameter (“OD”) with a dimension thatgenerally equal to the rotor core outer diameter. The end ring innerdiameter surface 106 has an inner diameter (“ID”) with a dimension thatgenerally corresponds to the rotor shaft outer diameter. The inner face100 abuts the laminations 40, and the outer face 102 may have featuresallowing the rotor to be balanced (i.e., drilled holes). The inner andouter faces 100,102 may be separated by a width (“w”). The outer andinner diameter surfaces may be separated by a height (“h”) which isequal to (ϕOD−ϕID)/2. Providing the end rings with an outer diametergenerally equal to the rotor core outer diameter produces favorablestress conditions in the end rings 46 when the rotor operates at ratedspeed. With the end ring outer diameter generally equal to or slightlyless than the rotor core outer diameter, the center of gravity of theend ring is sufficiently positioned toward the axis of rotation of therotor to reduce stress in the end plates while providing structuralintegrity for the rotor core without otherwise increasing the rotor coreinertia. The end ring outer diameter and inner diameter surface may betapered. With reference to FIG. 8, the end ring area (“ERA”) may beprovided by the equation ERA=[[(ϕOD−ϕID)×w]−[w²×(TAN(a)+TAN(b))]]/2.

While the laminations 40 forming the rotor core may be made fromelectrical steel, as is typical, the end rings 46 may be made from acopper material or an aluminum material, or other highly electricallyconductive metal. The conductivity (“σ”) associated with severalcommonly used materials for rotor bars and end rings is shown below. Forpurposes of discussion herein, materials with a conductivity of greaterthan 90% (IACS) are considered high conductivity materials.

Conductivity Base Alloy Material (IACs) Aluminum Aluminum Alloy 100.154% Aluminum Aluminum Alloy 130.1 55% Aluminum Aluminum Alloy 150.1 57%Aluminum Aluminum Alloy 170.1 59% Copper Copper Alloy C10100, 10200 101%Copper Copper Alloy 11000 101% Copper Copper Alloy C11300, C11400,C11600 100% Copper Electrolytic (ETP) 101% Copper Silver-bearing, 8 oz/t101% Copper Silver-bearing, 10 to 15 oz/t 101% Copper Silver-bearing, 25to 30 oz/t 101% Copper Oxygen-free (OF) 101% Copper Phosphorized (DLP)97% to 100%  Copper Free-cutting (S, Te or Pb) 90% to 98%   CopperChromium coppers 80 to 90% Copper Phosphorized (DHP) 80 to 90% CopperCadmium copper (1%) 80 to 90%As mentioned before, the rotor bars may be fabricated/swaged or may becast. One or both of the end rings may be fabricated and/or cast. Toallow a cast end ring to be removed from a mold, the ring outer andinner diameter surfaces may be tapered as shown in FIGS. 7 and 8. Therotor bars and end rings may be made from different materials withdifferent or similar conductivities. The rotor bars and end rings mayalso be made from the same material.

A decrease in asynchronous slip may be achieved when the rotor bars aremade from materials with favorable conductivity properties. Forinstance, for a given load and starting current, the asynchronous slipof a LSIPM may be decreased by forming its rotor bars from copper allowsrather than from aluminum alloys.

A decrease in asynchronous slip may be achieved when the minimumgeometric cross-sectional area of the end rings (“ERA”) is greater than0.5 times the rotor bar area (“BA”) per the number of poles (P) times aratio of the rotor bar material conductivity to the end ring materialconductivity (σ_(RB)/σ_(EM)). FIG. 9 provides a chart showing dataassociated with a 280 size frame, 4 poles/3 phase LSIPM with a 6″ corelength having a total rotor bar area (“BA”) of 4.0050468 in². The bararea per pole (BAP) was given by the equation (BAP=(BA/P)) and is shownas 1.001262. Because the rotor bars and the end rings were formed fromthe same material, the ratio of the rotor bar material conductivity tothe end member material conductivity (σ_(RB)/σ_(EM)) is 1. Each row ofthe chart of FIG. 9 corresponds to a rotor configuration comprising anend ring with the same outer diameter and inner diameter (i.e., the sameheight (“h”)) and a different width (“w”). The slip associated with eachconfiguration decreases as the ERA/BAP approaches 0.5. When ERA/BAPapproaches 1.3, slip is relatively constant. Likewise, the maximuminertia to be synchronized increases as the ERA/BAP approaches 0.5, andwhen ERA/BAP approaches 1.3, the maximum inertia to be synchronized isrelatively constant. In each case, the stator flux was essentially aconstant value with a starting current ranging from 269 to 273 amps. Forpurposes of discussion herein, an equivalent range includes variationsof up to and including 2.5% of starting current. As shown in thetabulated data of FIG. 9, a range of ERA/BAP of about ⅔ to about 2.0 forrotor bars and end rings having the same conductivity providesimprovements in inertia synchronization capability without significantimpact on starting current.

If end rings and bars are cast, there is a potential for porosity whichdecreases conductivity. Thus, the ratio of ERA/BAP may be selectedtoward the higher range to account for the decrease in effectiveconductivity of the end ring due to the higher porosity in the end ringthan the rotor bar. In a similar fashion, the ratio of the rotor barmaterial conductivity to the end member material conductivity(σ_(RB)/σ_(EM)) provides a factor to account for rotor bar materials andend ring materials having different material conductivities. Forinstance, where the rotor bars are made of copper and the end rings aremade of aluminum, the ratio of the conductivity of copper to theconductivity of aluminum is 1.772 (i.e., 101/57). A factor of 1.772 canthen be applied to ERA/BAP as desired. Thus, in the example of FIG. 9,in order for the motor to synchronize a load of 74 lb-ft², the minimumwidth of an aluminum end ring used with copper rotor bars would need tobe increased from 0.850 in. to 1.505 in. for the same diameter end ring.While the drawings show end rings having the same cross-sectionalgeometry, one end ring may vary dimensionally from its axially oppositeend ring, for instance, have a different width (w) dimension. However,for purposes of a design capable of synchronizing the higher inertia asdescribed above, the cross sectional area (“ERA”) of both end ringsshould be no less than about 0.5 times greater than the rotor bar areaper the number of poles (BAP) times a ratio of the rotor bar materialconductivity to the end member material conductivity (σ_(RB)/σ_(EM)),and preferably between about ⅔ and about 2 times greater than the rotorbar area per the number of poles (BAP) times a ratio of the rotor barmaterial conductivity to the end member material conductivity(σ_(RB)/σ_(EM)). As is seen in the foregoing, a LSIPM motor according tothe present teachings may drive a relatively higher inertial load fromstart to at least near synchronous speed during steady state operationof the motor when its ERA is greater than about 0.5 times the rotor bararea per the number of poles (BAP) times a ratio of the rotor barmaterial conductivity to the end member material conductivity(σ_(RB)/σ_(EM)), with other variables associated with the motor beingequal or remaining constant, for instance, starting current, startingtorque, bar configuration, magnet configuration, or other configurationsof the laminations.

The performance of an LSIPM during synchronous steady state operationmay be enhanced by maximizing the saturation boundary area and themagnet size. These considerations result in less lamination area beingavailable for rotor bars for a given size motor. Providing an end ringmember with a minimum geometric cross sectional area (“ERA”) that isgreater than about 0.5 times the rotor bar area per the number of poles(BAP) times a ratio of the rotor bar material conductivity to the endmember material conductivity (σ_(RB)/σ_(EM)) provides improvements inthe LSIPM's ability to synchronize loads with a relatively high inertia.For instance, a LSIPM having an end ring member with a minimum geometriccross sectional area (“ERA”) that is greater than about 0.5 times therotor bar area per the number of poles (BAP) times a ratio of the rotorbar material conductivity to the end member material conductivity(σ_(RB)/σ_(EM)) can synchronize a load with an inertia that is greaterthan the load the LSIPM motor can synchronize when the end ring memberhas a minimum geometric cross sectional area (“ERA”) that is less thanabout 0.5 times the rotor bar area per the number of poles (BAP) times aratio of the rotor bar material conductivity to the end member materialconductivity (σ_(RB)/σ_(EM)), for an equivalent range of startingcurrent. In another aspect of the teachings, providing a rotor cageformed from materials having conductivity greater than other materialsallows for improvements in the LSIPM's ability to synchronize loads witha relatively high inertia. For instance, a LSIPM having a rotor cageformed from a material having a first conductivity can synchronize aload with an inertia that is greater than the load the LSIPM motor cansynchronize when the rotor cage is formed from materials having aconductivity lower than the first conductivity and the motor issubjected to an equivalent range of starting current.

While certain embodiments have been described in detail in the foregoingdetailed description and illustrated in the accompanying drawings, thosewith ordinary skill in the art will appreciate that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Particularly, thefigures and exemplar embodiments of the rotor laminations are intendedto show illustrative examples and not to be considered limiting in anysense. Accordingly, the particular arrangements disclosed are meant tobe illustrative only and not limiting as to the scope of the inventionwhich is to be given the full breadth of the appended claims and any andall equivalents thereof.

The invention claimed is:
 1. A method comprising: providing a line-startsynchronous motor, wherein the motor comprises: a stator; a rotor coredisposed within the stator, the rotor core comprising: a plurality ofpermanent magnets defining a number of poles (P) for the machine; and aplurality of rotor bars spaced about the rotor core, the rotor barsdefining a rotor bar area (BA) and being formed of a conductive materialhaving an associated conductivity (σ_(RB)); and end members disposed onaxial opposite ends of the rotor core, the end members being inelectrical contact with the rotor bars, each end member having ageometric cross sectional area (“ERA”), wherein the end members have anouter diameter that generally corresponds to an outer diameter of therotor core, and the end members are formed from a material having anassociated conductivity (σ_(EM)); operatively coupling a first load tothe motor; and driving the first load from start to at least nearsynchronous speed during steady state operation of the motor with thefirst load coupled thereto; wherein the ERA of each of the end membersis greater than 0.5 times the rotor bar area per the number of poles(BA/P) times a ratio of the rotor bar material conductivity to the endmember material conductivity (σ_(RB)/σ_(EM)).
 2. The method of claim 1,wherein the first load has an inertia, the first load inertia beinggreater than an inertia of a second load, the second load inertia beinga maximum inertia the motor is capable of driving from start to nearsynchronous speed during steady state operation of the motor with thesecond load coupled thereto when the motor has a rotor with end memberswith a geometric cross sectional area that is less than about 0.5 timesthe rotor bar area per the number of poles (BA/P) times a ratio of therotor bar material conductivity to the end member material conductivity(σ_(RB)/σ_(EM)) and the motor is subjected to an equivalent range ofstarting current as the starting current applied to drive the first loadwhen herein the ERA of each of the end members is greater than 0.5 timesthe rotor bar area per the number of poles (BA/P) times a ratio of therotor bar material conductivity to the end member material conductivity(σ_(RB)/σ_(EM)).
 3. The method of claim 1, wherein the ERA is betweenabout ⅔ times and about 2 times greater than the rotor bar area per thenumber of poles (BA/P) times a ratio of the rotor bar materialconductivity to the end member material conductivity (σ_(RB)/σ_(EM)). 4.The method of claim 1, wherein each of the plurality of permanentmagnets is disposed in a corresponding magnet slot in the rotor and,together, define a general axis of magnetization of each pole of therotor, and wherein edges of the magnet slots that face the general axisof magnetization define a saturation boundary area.
 5. The method ofclaim 4, wherein the magnet slots are adjacent to the outer diameter ofthe rotor and extend generally inward toward a center axis of the rotorcore.
 6. The method of claim 5, wherein at least some of the rotor barslots are disposed in the saturation boundary area and spaced from themagnet slots.
 7. The method of claim 1, wherein the rotor core comprisesa plurality of generally like laminations stacked end to end to form acontiguous rotor core.
 8. The method of claim 1, wherein the motor isconfigured to attain synchronous speed after starting of the motor withthe first load operatively coupled thereto.
 9. The method of claim 1,wherein the end members have generally a same cross sectional area. 10.The method of claim 1, wherein the rotor bars and the end members areformed from a same base alloy.
 11. A method comprising: accessing aline-start synchronous motor, wherein the motor comprises: a stator; arotor core disposed within the stator, the rotor core comprising: aplurality of permanent magnets defining a number of poles (P) of themotor; a plurality of rotor bars spaced about the rotor core, the rotorbars defining a rotor bar area (BA), the rotor bars being formed of aconductive material having an associated conductivity (σ_(RB)); and endmembers disposed on axial opposite ends of the rotor core, the endmembers being in electrical contact with the rotor bars, each end memberhaving a geometric cross sectional area (“ERA”), wherein the end membershave a diameter that generally corresponds to an outer diameter of therotor core, and wherein the ERA is greater than 0.5 times the rotor bararea per the number of poles (BA/P) times a ratio of the rotor barmaterial conductivity to the end member material conductivity(σ_(RB)/σ_(EM)); operatively coupling a first load to the motor; anddriving the first load from start to at least near synchronous speedduring steady state operation of the motor with the first load coupledthereto; wherein the first load has an inertia, the first load inertiabeing greater than an inertia of a second load, the second load inertiabeing a maximum inertia that a second motor is capable of driving fromstart to near synchronous speed during steady state operation of thesecond motor with the second load coupled thereto when the second motorhas a rotor with end members with a geometric cross sectional area lessthan about 0.5 times the rotor bar area per the number of poles (BA/P)times a ratio of the rotor bar material conductivity to the end membermaterial conductivity (σ_(RB)/σ_(EM)) and the second motor is subjectedto an equivalent range of starting current as the motor.
 12. The methodof claim 11, wherein the ERA is between about ⅔ times and about 2 timesgreater than the rotor bar area per the number of poles (BA/P) times aratio of the rotor bar material conductivity to the end member materialconductivity (σ_(RB)/σ_(EM)).
 13. The method of claim 11, wherein theend members have a like geometry.
 14. The method of claim 11, whereinthe magnets are disposed in slots and define a general axis ofmagnetization of each pole of the rotor, and edges of the magnet slotsthat face the general axis of magnetization define a saturation boundaryarea.
 15. The method of claim 14, wherein the magnet slots are adjacentto the rotor outer diameter and extend generally inward toward a centeraxis of the rotor core.
 16. The method of claim 15, wherein at leastsome of the rotor bar slots are disposed in the saturation boundary areaand spaced from the magnet slots.
 17. The method of claim 11, whereinthe rotor core comprises a plurality of generally like laminationsstacked end to end to form a contiguous rotor core.
 18. The method ofclaim 11, wherein the rotor bars and the end members are formed from asame base alloy.
 19. The method of claim 11, wherein each of the endmembers is ring shaped.
 20. The method of claim 1, wherein each of theend members is ring shaped.